This invention relates to polypeptide hormone analogues that exhibits enhanced pharmaceutical properties, such as more rapid pharmacokinetics and/or augmented resistance to thermal fibrillation above room temperature. More particularly, this invention relates to insulin analogues that are modified by deletion of residues B1-B3 in combination with other modifications of the A-chain or B-chain. Such non-standard sequences may optionally contain standard amino-acid substitutions at other sites in the A or B chains of an insulin analogue.
The engineering of truncated and/or non-standard proteins, including therapeutic agents and vaccines, may have broad medical and societal benefits. An example of a medical benefit would be optimization of the pharmacokinetic properties of a protein. An example of a further societal benefit would be the engineering of proteins more refractory than standard proteins with respect to degradation at or above room temperature for use in regions of the developing world where electricity and refrigeration are not consistently available. An example of a therapeutic protein is provided by insulin. Analogues of insulin containing non-standard amino-acid substitutions may in principle exhibit superior properties with respect to pharmacokinetics or resistance to thermal degradation. The challenge posed by the pharmacokinetics of insulin absorption following subcutaneous injection affects the ability of patients to achieve tight glycemic control and constrains the safety and performance of insulin pumps. The challenge posed by its physical degradation is deepened by the pending epidemic of diabetes mellitus in Africa and Asia. These issues are often coupled as modifications known in the art to accelerate absorption following subcutaneous injection usually worsen the resistance of insulin to chemical and/or physical degradation. Because fibrillation poses the major route of degradation above room temperature, the design of fibrillation-resistant formulations may enhance the safety and efficacy of insulin replacement therapy in such challenged regions. The present invention pertains to the use of a truncated B-chain lacking residues B1-B3 in combination with standard or non-standard substitutions elsewhere in the A-chain or B-chain to modify and improve distinct properties of insulin. During the past decade specific chemical modifications to the insulin molecule have been described that selectively modify one or another particular property of the protein to facilitate an application of interest. Whereas at the beginning of the recombinant DNA era (1980) wild-type human insulin was envisaged as being optimal for use in diverse therapeutic contexts, the broad clinical use of insulin analogues in the past decade suggests that a suite of analogs, each tailored to address a specific unmet need, would provide significant medical and societal benefits.
Administration of insulin has long been established as a treatment for diabetes mellitus. Insulin is a small globular protein that plays a central role in metabolism in vertebrates. Insulin contains two chains, an A chain, containing 21 residues, and a B chain containing 30 residues. The hormone is stored in the pancreatic β-cell as a Zn2+-stabilized hexamer, but functions as a Zn2+-free monomer in the bloodstream. Insulin is the product of a single-chain precursor, proinsulin, in which a connecting region (35 residues) links the C-terminal residue of B chain (residue B30) to the N-terminal residue of the A chain (FIG. 1A). A variety of evidence indicates that it consists of an insulin-like core and disordered connecting peptide (FIG. 1B). Formation of three specific disulfide bridges (A6-A11, A7-B7, and A20-B19; FIGS. 1A and 1B) is thought to be coupled to oxidative folding of proinsulin in the rough endoplasmic reticulum (ER). Proinsulin assembles to form soluble Zn2+-coordinated hexamers shortly after export from ER to the Golgi apparatus. Endoproteolytic digestion and conversion to insulin occurs in immature secretory granules followed by morphological condensation. Crystalline arrays of zinc insulin hexamers within mature storage granules have been visualized by electron microscopy (EM). The sequence of insulin is shown in schematic form in FIG. 1C. Individual residues are indicated by the identity of the amino acid (typically using a standard three-letter code), the chain and sequence position (typically as a superscript).
The pharmacokinetic features of insulin absorption after subcutaneous injection have been found to correlate with the rate of disassembly of the insulin hexamer. Although not wishing the present invention to be constrained by theory, modifications to the insulin molecule that lead to accelerated disassembly of the insulin hexamer are thought to promote more rapid absorption of insulin monomers and dimers from the subcutaneous depot into the bloodstream. A major goal of insulin replacement therapy in patients with diabetes mellitus is tight control of the blood glucose concentration to prevent its excursion above or below the normal range characteristic of healthy human subjects. Excursions below the normal range are associated with immediate adrenergic or neuroglycopenic symptoms, which in severe episodes lead to convulsions, coma, and death. Excursions above the normal range are associated with increased long-term risk of microvascular disease, including retinapathy, blindness, and renal failure. Because the pharmacokinetics of absorption of wild-type human insulin following subcutaneous injection is often too slow and too prolonged relative to the physiological requirements of post-prandial metabolic homeostasis, considerable efforts have been expended during the past 20 years to develop insulin analogues that exhibit more rapid absorption with pharmacodynamic effects that are more rapid in onset and less prolonged in duration. Examples of such rapid-acting analogues known in the art are [LysB28, ProB29]-insulin (KP-insulin, the active component of Humalog®), [AspB28]-insulin (Novalog®), and [LysB3, GluB29]-insulin (Apidra®). Although widely used in clinical practice, these analogues exhibit two principal limitations. First, although their pharmacokinetic and pharmacodynamic profiles are more rapid than those of wild-type insulin, they are not rapid enough in many patients to optimize glycemic control or enable the safe and effective use of algorithm-based insulin pumps (closed-loop systems). Second, the amino-acid substitutions in these analogues impair the thermodynamic stability of insulin and exacerbate its susceptibility to fibrillation above room temperature. Thus, the safety, efficacy, and real-world convenience of these products have been limited by a trade-off between accelerated absorption and accelerated degradation.
Fibrillation, which is a serious concern in the manufacture, storage and use of insulin and insulin analogues for the treatment of diabetes mellitus, is enhanced with higher temperature, lower pH, agitation, or the presence of urea, guanidine, ethanol co-solvent, or hydrophobic surfaces. Current US drug regulations demand that insulin be discarded if fibrillation occurs at a level of one percent or more. Because fibrillation is enhanced at higher temperatures, patients with diabetes mellitus optimally must keep insulin refrigerated prior to use. Fibrillation of insulin or an insulin analogue can be a particular concern for such patients utilizing an external insulin pump, in which small amounts of insulin or insulin analogue are injected into the patient's body at regular intervals. In such a usage, the insulin or insulin analogue is not kept refrigerated within the pump apparatus, and fibrillation of insulin can result in blockage of the catheter used to inject insulin or insulin analogue into the body, potentially resulting in unpredictable fluctuations in blood glucose levels or even dangerous hyperglycemia. At least one recent report has indicated that insulin Lispro (KP-insulin, an analogue in which residues B28 and B29 are interchanged relative to their positions in wild-type human insulin; trade name Humalog®) may be particularly susceptible to fibrillation and resulting obstruction of insulin pump catheters. Insulin exhibits an increase in degradation rate of 10-fold or more for each 10° C. increment in temperature above 25° C.; accordingly, guidelines call for storage at temperatures <30° C. and preferably with refrigeration.
The present theory of protein fibrillation posits that the mechanism of fibrillation proceeds via a partially folded intermediate state, which in turn aggregates to form an amyloidogenic nucleus. In this theory, it is possible that amino-acid substitutions that stabilize the native state may or may not stabilize the partially folded intermediate state and may or may not increase (or decrease) the free-energy barrier between the native state and the intermediate state. Therefore, the current theory indicates that the tendency of a given amino-acid substitution in the insulin molecule to increase or decrease the risk of fibrillation is highly unpredictable.
Initial steps in the formation of an amyloidogenic nucleus may involve non-native hydrophobic interactions between nonpolar side chains that are either exposed on the surface of native insulin or are transiently exposed due to partial protein unfolding. Residues B1, B2, and B3 (PheB1-ValB2-AsnB3) exhibit significant exposure in crystal structures of wild-type insulin and substantial variability in their positioning (FIG. 2A). Residues B1-B3 are also less well ordered in NMR studies of engineered insulin monomers in solution as indicated by 1H-NMR resonance line widths, 13C-NMR chemical shifts, and paucity of long-range inter-residue nuclear Overhauser effects (NOEs) (FIG. 2B). Despite the conservation of these residues among mammalian insulin sequences, studies of synthetic analogs has shown that residues B1-B5 may be deleted from the B-chain without significant loss of binding to the insulin receptor and without significant loss of biological activity. Studies of the folding and secretion of wild-type and mutant proinsulins have nonetheless shown that residues B1-B5, singly and in combination, contribute to the efficiency and fidelity of disulfide pairing in the endoplasmic reticulum (Liu, M., Hua, Q. X., Hu, S. Q., Jia, W., Yang, Y., Saith, S. E., Whittaker, J., Aryan, P. & Weiss, M. A. (2010)). Deciphering the hidden informational content of protein sequences: foldability of proinsulin hinges on a flexible arm that is dispensable in the mature hormone. J. Biol. Chem. 285, 30989-31001). In particular, diverse polar or charged amino-acid substitutions at position B1 have been found to impair the cellular folding and secretion of proinsulin in a human cell line in culture. Residue PheB1 may therefore be regarded as a “hydrophobic anchor” in the folding process. The B1-B5 arm of the B-chain represents a key folding element within the pancreatic beta-cell.
There is a need, therefore for an insulin analogue that displays increased resistance to fibrillation above room temperature while retaining rapid hexamer disassembly and while exhibiting at least a portion of the activity of the corresponding wild-type insulin and maintaining at least a portion of its chemical and/or physical stability.